In a typical radio communications network, wireless terminals, also known as mobile stations, terminals and/or user equipments, UEs, communicate via a Radio Access Network, RAN, to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g. a radio base station, RBS, or network node, which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System, UMTS, is a third generation mobile communication system, which evolved from the second generation, 2G, Global System for Mobile Communications, GSM. The UMTS terrestrial radio access network, UTRAN, is essentially a RAN using wideband code division multiple access, WCDMA, and/or High Speed Packet Access, HSPA, for user equipments. In a forum known as the Third Generation Partnership Project, 3GPP, telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller, RNC, or a base station controller, BSC, which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System, EPS, have been completed within the 3rd Generation Partnership Project, 3GPP, and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network, E-UTRAN, also known as the Long Term Evolution, LTE, radio access, and the Evolved Packet Core, EPC, also known as System Architecture Evolution, SAE, core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network, RAN, of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
In LTE, as in any communication system, a UE may estimate the effective channel that a reference signal is traversing by measuring on the reference signal, e.g. a Channel State Information Resource Symbol, CSI-RS, defined for LTE. Here, the effective channel comprises the radio propagation channel, antenna gains, and any possible antenna virtualizations. For antenna virtualization, a CSI-RS port may be precoded so that it is virtualized over multiple physical antenna ports, that is, the CSI-RS port may be transmitted on multiple physical antenna ports, possibly with different gains and phases.
By further configuring radio resources which a UE is mandated to use for measuring interference plus noise, such as, for example, CSI-Interference Management, CSI-IM, resources defined for LTE, a UE may assume that there are a number of transmission points that are transmitting on this radio resource, and that the received signal power may therefore be used as a measurement of the interference plus noise from these transmission points. Thus, based on a specified reference signal measurement and on an interference measurement configuration, the UE may estimate the effective channel and interference plus noise, and consequently also determine which rank, pre-coder and transport format to recommend which best matches the particular channel.
The CSI resource feedback from the UE to the network node may be either explicit or implicit. LTE has currently adopted an implicit CSI mechanism in which a UE does not explicitly report, e.g. the complex valued elements of a measured effective channel, but rather that the UE recommends a transmission configuration suitable for the measured effective channel. The recommended transmission configuration thus implicitly gives the network node information about the underlying channel state.
In LTE, the CSI feedback is given in terms of a transmission Rank Indicator, RI, a Pre-coder Matrix Indicator, PMI, and Channel Quality Indicator(s), CQI. The CQI/RI/PMI report, i.e. the CSI report, may be wideband or frequency selective depending on which reporting mode that is configured. Typically, the UE performs filter processing of the measured reference signals as a means to improve receiver performance by the UE. This filter processing may be performed in time and frequency, and in some cases, such as for De-Modulation Reference Signals, DMRS, the UE is mandated to perform the filter processing according to standard specifications.
However, for CSI feedback reports, there is no mandated filter processing of the interference measurements performed on CSI-IM. Thus, the filter processing of the interference measurements performed on CSI-IM by the UE is a UE receiver design choice that is proprietary for each UE vendor. Typically, a UE performs time filter processing of the measured interference on CSI-IM, and use the time filtered value when calculating the CQI, RI and PMI to include in the CSI reports.
It may be noted that the RI corresponds to a recommended number of streams that are to be spatially multiplexed and thus transmitted in parallel over the effective channel, whereas the PMI identifies a recommended pre-coder, normally present in a codebook, for the transmission, which relates to the spatial characteristics of the effective channel.
The CQI represents a recommended Modulation and Coding Scheme, MCS. For example, a UE may normally report one of 16 different CQI values representing 16 different MCS. The UE reports the highest CQI value that has target block error rate less than 10%. Thus, since the current Signal-to-Interference-plus-Noise Ratio, SINR, of the spatial stream(s) over which the transmission occurs directly effects the target block error rate, there is thus a relationship between the CQI and the SINR of the spatial stream(s) over which the transmission occurs.
In uncoordinated systems, i.e. wherein each transmission point or cell independently performs transmissions to the UEs located within its range, the UE may effectively measure the interference observed from all other transmission points, or all other cells, using wideband interference information, such as, for example, Reference Signal Received Power (RSRP). This may then serve as the relevant interference level in an upcoming data transmission.
In coordinated systems, i.e. wherein multiple transmission points or cells may schedule and perform coordinated transmissions to the UEs located within their ranges, the network may to a large extent control the transmission points or cells that are interfering with transmissions to a UE. Hence, there will here be multiple interference hypotheses/scenarios which will depend on which transmissions points or cells that are transmitting data to other UEs.
Additionally, the network may here also choose to transmit interference from specific transmission points or cells for the purpose of testing how that particular interference hypothesis/scenario effects transmissions to a UE.
In 3GPP LTE Release 11, CSI processes are defined such that each CSI process is associated with a CSI-RS resource and a CSI-IM resource (see e.g. 3GPP TS 36.213 V11.4.0, 2013-09, Sections 7.2.5-7.2.6). A UE configured for Transmission Mode 10, TM10, in LTE Release 11 may be configured with one or more CSI processes per serving cell by higher layers, and a CSI report reported by the UE corresponds to a CSI process. Since multiple, e.g. up to three or perhaps even six, CSI processes may be reported by the UEs, the network may test different interference hypotheses or scenarios simultaneously for a UE. Then, based on their different effects which are reported back to the network node by the UE in the CSI reports, the network node may adapt its transmission scheme to the UE for future transmissions.
For large coordination clusters in challenging scenarios with many strong interferers, the network may require CSI information corresponding to many different interference hypotheses/scenarios. Depending on the coordination scheme, the number of interference hypotheses/scenarios to test may be as many as 2N for N number of interferers. A known way to assess multiple interference hypotheses/scenarios is to use time-multiplexing, i.e. change the interference hypotheses/scenarios in time and store the results. This, however, may be a time-consuming task and result in that the interference measurement may not be performed on a fast enough basis for coordinated transmissions.